The Research Of Measurement In NMR-LWD

NMR-LWD overcomes the application limitations of cable logging and ignores theinvasion of drilling fluid. It guides drilling process in producing formation, enlarges theexposed area of the fluid in bore, enhances production and recovery, thus reduces the costof drilling. Due to the collision between the drill bit and the rock, the instrument movesrandomly and the result of CPMG pulse sequence logging has a bad quality. By studyingthe moving characteristics of the instrument, the logging process can be correctly directedand the logging quality can be effectively improved.Thisthesis discusses and analyses the instrument movement problem. First, themechanical analysis of the instrument is conducted and the4forms of movement arestudied. Based on the principle of NMR-LWD, the cross movement is determined as themain factor which affects the logging quality. Second, a method by using inertiameasurement meter is proposed to measure the cross movement in real time. The theoryof measuring cross movement by using axial symmetrically installed dual axisaccelerometer and magnetometer is analysed. So is the algorithm of calculatingdisplacement by using acceleration and angle. At last, by acquiring movingcharacteristics in real time, three methods are provided to reduce the influence ofinstrument movement on echo signal: by establishing low gradient static magnetic field,by adjusting pulse sequence according to the moving conditions and by compensatingecho signal according to the measured displacement.Based on the measurement theory and method above, a cross movement detectioncircuit is developed. This circuit is mainly based on DSP and FPGA framework. With thehelp of sensors and signal conditioning circuits, it realizes movement data acquisition,data processing and transmission. This measurement system is real-time, small in size,high temperature and vibration resistant. It is very suitable to be applied in NMR-LWD.